Order Email Problem

[jsmekerall9320]

Going thru testing of my site and about to go live soon, a new problem appeared and I'm stumped as to the cause.  When an order is placed thru the site, the order confirmation emails are being flagged at my email host, and then bounced to the sender setup in Magento (me).  The error received is:

550 Messages should have one or no Content-Type headers, not 2.  

All other emails work fine.  New user registration, order invoice, order shipping.  Just not the order confirmation.

Now, I am working the BSSCommerce on an extension that adds attributes to the body of some emails, but they swear their extension is not the cause of this issue.  I can confirm, that disabling that extension, the problem persists.

I'm using custom email templates, but even disabling those, and using the default emails, I have the same problem.

Really stumped here.  Any ideas on what could cause this problem?  Or what I can check?  Thanks so much for the help!

Using Magento CE 2.3.4

[shubham_khandelwal1]

Hi @jsmekerall9320 

Hope you are doing good,

I have faced this issue a long time ago so what i did is i disable the 3rd party extension and then track this issue but at the end issue still persist then i check with the 3rd party email host plugin and issue was there. So in order to check your issue can you please check weather your plugin is updated or not? Because this can cause some issue.

Also if above solution won't work then please disable extension one by one and check the issue.

Hope this help.

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[tommyquiss0041]

You could try setting the "Send Order Email Copy Method" to 
"Separate Email". This option can be found under "Stores" > "Configuration" > "Sales" > "Sales Emails" > "Order"